Changes of Sucrose-Phosphate Synthase Activity in Barley Primary Leaves during Light/Dark Transitions

Carbon and nitrogen metabolism affects kentucky bluegrass rhizome expansion

BACKGROUND

Rhizome is vital for carbon and nitrogen metabolism of the whole plant. However, the effect of carbon and nitrogen in the rhizome on rhizome expansion remains unclear.

RESULTS

Three wild Kentucky bluegrass (Poa pratensis L.) germplasms with different rhizome expansion capacity (strong expansion capacity, 'YZ'; medium expansion capacity, 'WY'; and weak expansion capacity, 'AD') were planted in the field and the rhizomes number, tiller number, rhizome dry weight, physiological indicators and enzyme activity associated carbon and nitrogen metabolisms were measured. Liquid chromatography coupled to mass spectrometry (LC-MS) was utilized to analyze the metabolomic of the rhizomes. The results showed that the rhizome and tiller numbers of the YZ were 3.26 and 2.69-fold of that of the AD, respectively. The aboveground dry weight of the YZ was the greatest among all three germplasms. Contents of soluble sugar, starch, sucrose, NO₃^--N, and free amino acid were significantly higher in rhizomes of the YZ than those of the WY and AD (P < 0.05). The activities of glutamine synthetase (GS), glutamate dehydrogenase (GDH) and sucrose phosphate synthase (SPS) of the YZ were the highest among all three germplasm, with values of 17.73 A·g^- 1 h^- 1, 5.96 µmol·g^- 1 min^- 1, and 11.35 mg·g^- 1 h^- 1, respectively. Metabolomics analyses revealed that a total of 28 differentially expressed metabolites (DEMs) were up-regulated, and 25 DEMs were down-regulated in both comparison groups (AD vs. YZ group and WY vs. YZ group). Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis demonstrated that metabolites related to histidine metabolism, tyrosine metabolism, tryptophan metabolism, and phenylalanine metabolism were associated with rhizomes carbon and nitrogen metabolism.

CONCLUSIONS

Overall, the results suggest that soluble sugar, starch, sucrose, NO₃^--N, and free amino acid in rhizome are important to and promote rhizome expansion in Kentucky bluegrass, while tryptamine, 3-methylhistidine, 3-indoleacetonitrile, indole, and histamine may be key metabolites in promoting carbon and nitrogen metabolism of rhizome.

The Metabolic Signature of Biomass Formation in Barley

The network analysis of genome-wide transcriptome responses, metabolic signatures and enzymes' relationship to biomass formation has been studied in a diverse panel of 12 barley accessions during vegetative and reproductive stages. The primary metabolites and enzymes involved in central metabolism that determine the accumulation of shoot biomass at the vegetative stage of barley development are primarily being linked to sucrose accumulation and sucrose synthase activity. Interestingly, the metabolic and enzyme links which are strongly associated with biomass accumulation during reproductive stages are related to starch accumulation and tricarboxylic acid (TCA) cycle intermediates citrate, malate, trans-aconitate and isocitrate. Additional significant associations were also found for UDP glucose, ATP and the amino acids isoleucine, valine, glutamate and histidine during the reproductive stage. A network analysis resulted in a combined identification of metabolite and enzyme signatures indicative for grain weight accumulation that was correlated with the activity of ADP-glucose pyrophosphorylase (AGPase), a rate-limiting enzyme involved in starch biosynthesis, and with that of alanine amino transferase involved in the synthesis of storage proteins. We propose that the mechanism related to vegetative and reproductive biomass formation vs. seed biomass formation is being linked to distinct fluxes regulating sucrose, starch, sugars and amino acids as central resources. These distinct biomarkers can be used to engineer biomass production and grain weight in barley.

Involvement of 14-3-3 protein GRF9 in root growth and response under polyethylene glycol-induced water stress

Plant 14-3-3 proteins are phosphoserine-binding proteins that regulate a wide array of targets via direct protein-protein interactions. In this study, the role of a 14-3-3 protein, GRF9, in plant response to water stress was investigated. Arabidopsis wild-type, GRF9-deficient mutant (grf9), and GRF9-overexpressing (OE) plants were treated with polyethylene glycol (PEG) to induce mild water stress. OE plant showed better whole-plant growth and root growth than the wild type under normal or water stress conditions while the grf9 mutant showed worse growth. In OE plants, GRF9 favours the allocation of shoot carbon to roots. In addition, GRF9 enhanced proton extrusion, mainly in the root elongation zone and root hair zone, and maintained root growth under mild water stress. Grafting among the wild type, OE, and grf9 plants showed that when OE plants were used as the scion and GRF9 was overexpressed in the shoot, it enhanced sucrose transport into the root, and when OE plants were used as rootstock and GRF9 was overexpressed in the root, it caused more release of protons into the root surface under water stress. Taken together, the results suggest that under PEG-induced water stress, GRF9 is involved in allocating more carbon from the shoot to the root and enhancing proton secretion in the root growing zone, and this process is important for root response to mild water stress.

TFT6 and TFT7, two different members of tomato 14-3-3 gene family, play distinct roles in plant adaption to low phosphorus stress

14-3-3 proteins are a large family of proteins but exact roles of their members in plant response to abiotic stresses are not clear, especially under nutrient deficiency. We investigated the expressions of all the tomato 14-3-3 gene family members (TFT1-TFT12) under low phosphorus stress (LP) and found that TFT6 belongs to the later responsive gene while TFT7 belongs to the early responsive gene. When the two genes were separately introduced into Arabidopsis and overexpressed, their plant growth under LP was much enhanced compared with wild-type plant. TFT6 overexpressing plants showed reduced starch synthase activity, reduced starch content but enhanced sucrose loading into phloem in the shoot under LP. TFT7 overexpressing plants had much enhanced H⁺ flux along their root tip and activity of plasma membrane H⁺-ATPase in the roots under LP. Our results suggest that TFT6 and TFT7 play different roles in plant adaption to LP. TFT6 acts mainly in leaves and is involved in the systemic response to LP by regulating leaf carbon allocation and increasing phloem sucrose transport to promote root growth, while TFT7 directly functions in root by activating root plasma membrane H⁺-ATPase to release more protons under LP.

The importance of maltose in transitory starch breakdown

Starch content of leaves responds to environmental stresses in various ways. Understanding these environmental effects on starch metabolism has been difficult in the past because the pathways of transitory starch synthesis and degradation are not completely known. Over the past two years there has been a significant increase in our understanding of transitory starch breakdown. The discovery of a maltose transporter (MEX1) and the studies of a cytosolic disproportionating enzyme (D-enzyme, DPE2) confirmed that maltose is the predominant form of carbon exported from chloroplasts at night. Maltose increases in leaves when starch breakdown is induced during the day under photorespiratory conditions. Maltose metabolism is regulated by a circadian clock, day length and temperature. The expression of maltose-metabolizing genes shows a pronounced circadian rhythm indicating maltose metabolism is clock regulated. Indeed, the maltose level oscillates under continuous light. The transcript of a beta-amylase gene (BAM3) peaks during the day in long days and peaks at night in short days. This could provide a mechanism for adjusting starch breakdown rates to day length. Under cold-stress conditions, maltose increases and BAM3 expression is induced. We hypothesize that maltose metabolism is a bridge between transitory starch breakdown and the plants' adaptation to changes in environmental conditions.

Light regulation of sucrose-phosphate synthase activity in the freezing-tolerant grass Deschampsia antarctica

Deschampsia antarctica, a freezing-tolerant grass that has colonized the Maritime Antarctic, has an unusually high content of sucrose (Suc) in leaves, reaching up to 36% of dry weight. Suc accumulation has often been linked with increased activity of sucrose phosphate synthase (SPS; EC: 2.4.1.1.14). SPS, a key enzyme in sucrose biosynthesis, is controlled by an intricate hierarchy of regulatory mechanisms including allosteric modulators, reversible covalent modification in response to illumination, and transcriptional regulation. We hypothesized that during long day conditions in the Antarctic summer D. antarctica can maintain high SPS activity longer by indirect light regulation, thereby leading to a high sucrose accumulation in the leaves. The objectives of this study were to investigate a possible indirect light regulation of SPS activity and the effect of cold and day length on transcriptional and protein level of SPS in D. antarctica. Although SPS activity did not display an endogenous rhythm of activity in continuous light, activation of SPS at the end of the dark period was observed in D. antarctica. This activation of SPS is possibly controlled by covalent modification, because it was inhibited by okadaic acid while the SPS protein level did not significantly change. The highest SPS activity increase was observed after 21 days of cold-acclimation under long day conditions. This increased activity was not related to an increase in SPS gene expression or protein content. High SPS activity in cold long days leading to hyper accumulation of Suc appears to be among the features that permit D. antarctica to survive in the harsh Antarctic conditions.

Carbon Partitioning in Eelgrass (Regulation by Photosynthesis and the Response to Daily Light-Dark Cycles)

Diel variations in rates of C export, sucrose-phosphate synthase (SPS) and sucrose synthase (SS) activity, and C reserves were investigated in Zostera marina L. (eelgrass) to elucidate the environmental regulation of sucrose formation and partitioning in this ecologically important species. Rates of C flux and SPS activity increased with leaf age, consistent with the ontogenic transition from sink to source status. Rates of C export and photosynthesis were low but quantitatively consistent with those of many terrestrial plant species. The Vmax activity of SPS approached that of maize, but substrate-limited rates were 20 to 25% of Vmax, indicating a large pool of inactive SPS. SPS was unresponsive to the day/night transition or to a 3-fold increase in photosynthesis generated by high [CO2] and showed little sensitivity to inorganic phosphate. Consequently, regulation of eelgrass SPS appeared similar to starch- rather than to sugar-accumulating species even though eelgrass accumulates sucrose. Leaf [sucrose] was constant and high throughout the diel cycle, which may contribute to the down-regulation of SPS. Root sucrose synthase activity was high but showed no response to nocturnal anoxia. Root [sucrose] also showed no diel cycle. The temporal stability of [sucrose] confers an ability for eelgrass to buffer the effects of prolonged light limitation that may be key to its survival and ecological success in environments subject to periods of extreme light limitation and chaotic daily variation in light availability.

In Vivo Regulation of Wheat-Leaf Phosphoenolpyruvate Carboxylase by Reversible Phosphorylation

Regulation of C3 phosphoenolpyruvate carboxylase (PEPC) and its protein-serine/threonine kinase (PEPC-PK) was studied in wheat (Triticum aestivum) leaves that were excised from low-N-grown seedlings and subsequently illuminated and/or supplied with 40 mM KNO3. The apparent phosphorylation status of PEPC was assessed by its sensitivity to L-malate inhibition at suboptimal assay conditions, and the activity state of PEPC-PK was determined by the in vitro 32P labeling of purified maize dephospho-PEPC by [[gamma]-32P]ATP/Mg. Illumination ([plus or minus]NO3-) for 1 h led to about a 4.5-fold increase in the 50% inhibition constant for L-malate, which was reversed by placing the illuminated detached leaves in darkness (minus NO3-). A 1 -h exposure of excised leaves to light, KNO3, or both resulted in relative PEPC-PK activities of 205, 119, and 659%, respectively, of the dark/0 mM KNO3 control tissue. In contrast, almost no activity was observed when a recombinant sorghum phosphorylation-site mutant (S8D) form of PEPC was used as protein substrate in PEPC-PK assays of the light plus KNO3 leaf extracts. In vivo labeling of wheat-leaf PEPC by feeding 32P-labeled orthophosphate showed that PEPC from light plus KNO3 tissue was substantially more phosphorylated than the enzyme in the dark minus-nitrate immunoprecipitates. Immunoblot analysis indicated that no changes in relative PEPC-protein amount occurred within 1 h for any of the treatments. Thus, C3 PEPC activity in these detached wheat leaves appears to be regulated by phosphorylation of a serine residue near the protein's N terminus by a Ca2+ -independent protein kinase in response to a complex interaction in vivo between light and N.

Adaptations of Photosynthetic Electron Transport, Carbon Assimilation, and Carbon Partitioning in Transgenic Nicotiana plumbaginifolia Plants to Changes in Nitrate Reductase Activity

Transgenic Nicotiana plumbaginifolia plants that express either a 5-fold increase or a 20-fold decrease in nitrate reductase (NR) activity were used to study the relationships between carbon and nitrogen metabolism in leaves. Under saturating irradiance the maximum rate of photosynthesis, per unit surface area, was decreased in the low NR expressors but was relatively unchanged in the high NR expressors compared with the wild-type controls. However, when photosynthesis was expressed on a chlorophyll (Chl) basis the low NR plants had comparable or even higher values than the wild-type plants. Surprisingly, the high NR expressors showed very similar rates of photosynthesis and respiration to the wild-type plants and contained identical amounts of leaf Chl, carbohydrate, and protein. These plants were provided with a saturating supply of nitrate plus a basal level of ammonium during all phases of growth. Under these conditions overexpression of NR had little impact on leaf metabolism and did not stimulate growth or biomass production. Large differences in photochemical quenching and nonphotochemical quenching components of Chl a fluorescence, as well as the ratio of variable to maximum fluorescence, (FV/FM), were apparent in the low NR expressors in comparison with the wild-type controls. Light intensity-dependent increases in nonphotochemical quenching and decreases in FV/FM were greatest in the low NR expressors, whereas photochemical quenching decreased uniformly with increasing irradiance in all plant types. Nonphotochemical quenching was increased at all except the lowest irradiances in the low NR expressors, allowing photosystem II to remain oxidized on its acceptor side. The relative contributions of photochemical and nonphotochemical quenching of Chl a fluorescence with changing irradiance were virtually identical in the high NR expressors and the wild-type controls. Zeaxanthin was present in all leaves at high irradiances; however, at high irradiance leaves from the low NR expressors contained considerably more zeaxanthin and less violaxanthin than wild-type controls or high NR expressors. The leaves of the low NR expressors contained less Chl, protein, and amino acids than controls but retained more carbohydrate (starch and sucrose) than the wild type or high NR expressors. Sucrose phosphate synthase activities were remarkably similar in all plant types regardless of the NR activity. In contrast phosphoenolpyruvate carboxylase activities were increased on a Chl or protein basis in the low NR expressors compared with the wild-type controls or high NR expressors. We conclude that large decreases in NR have profound repercussions for photosynthesis and carbon partitioning within the leaf but that increases in NR have negligible effects.

Mitochondrial Contribution to Photosynthetic Metabolism (A Study with Barley (Hordeum vulgare L.) Leaf Protoplasts at Different Light Intensities and CO2 Concentrations)

An oligomycin concentration that specifically inhibits oxidative phosphorylation was added to isolated barley (Hordeum vulgare L.) leaf protoplasts at various irradiances and carbon dioxide concentrations. At saturating as well as low light intensities, photosynthetic oxygen evolution was decreased as a result of the oligomycin treatment, whereas no effect was observed at intermediate light intensities. This was the same for photorespiratory and nonphotorespiratory conditions. These results were confirmed by measurements of fluorescence quenching under the same conditions. Metabolite analysis in the presence of oligomycin revealed a drastic decrease in the mitochondrial and cytosolic ATP/ADP ratios, whereas there was little or no effect on the chloroplastic ratio. Concomitantly, sucrose phosphate synthase activity was reduced. Under high irradiances, this inhibition of sucrose synthesis by oligomycin apparently caused a feedback inhibition on the Calvin cycle and the photosynthetic activity. Under low irradiances, a feedback regulation compensated, indicating that light was more limiting than the activity of regulative enzymes. Thus, the importance of mitochondrial respiratory activity might be different in different metabolic situations. At saturating light, the oxidation of excess photosynthetic redox equivalents is required to sustain a high rate of photosynthesis. At low light, the supply of ATP to the cytosol might be required to support biosynthetic reactions.

Regulation of sucrose phosphate synthase by gibberellins in soybean and spinach plants

Exogenous applications of gibberellins (GAs) increased the extractable activity of leaf sucrose phosphate synthase (SPS) in soybean (Glycine max [L.]) and spinach (Spinacia oleracea [L.]). The response to GA applications was detectable within 2 h postapplication and was still observed 6 h, 24 h, and 7 d after treatment. When paclobutrazol, a GA biosynthesis inhibitor, was applied to intact soybean and spinach plants, decreased extractable SPS activity resulted within 24 h following the treatment. Different methods of GA application (spray, injection, capillary wick, and excised leaf systems) produced similar effects on SPS activity of soybean leaves. Protein synthesis in soybean leaves appeared to be necessary for GA-promoted SPS activity because gibberellic acid only partially reversed the inhibitory effect of pretreatment with cycloheximide. Levels of SPS protein from crude extracts of spinach plants were measured by a dot blot technique using monoclonal antibodies against SPS. Application of gibberellic acid to spinach leaves increased levels of SPS protein 2 h, 24 h, and 7 d after treatment. The results suggest that, in both soybean and spinach, GA is one of the endogenous hormonal factors that regulate the steady-state level of SPS protein and, hence, its activity.

Nitrate activation of cytosolic protein kinases diverts photosynthetic carbon from sucrose to amino Acid biosynthesis: basis for a new concept

The regulation of carbon partitioning between carbohydrates (principally sucrose) and amino acids has been only poorly characterized in higher plants. The hypothesis that the pathway of sucrose and amino acid biosynthesis compete for carbon skeletons and energy is widely accepted. In this review, we suggest a mechanism involving the regulation of cytosolic protein kinases whereby the flow of carbon is regulated at the level of partitioning between the pathways of carbohydrate and nitrogen metabolism via the covalent modulation of component enzymes. The addition of nitrate to wheat seedlings (Triticum aestivum) grown in the absence of exogenous nitrogen has a dramatic, if transient, impact on sucrose formation and on the activities of sucrose phosphate synthase (which is inactivated) and phosphoenolpyruvate carboxylase (which is activated). The activities of these two enzymes are modulated by protein phosphorylation in response to the addition of nitrate, but they respond in an inverse fashion. Sucrose phosphate synthase in inactivated and phosphoenolpyruvate carboxylase is activated. Nitrate functions as a signal metabolite activating the cytosolic protein kinase, thereby modulating the activities of at least two of the key enzymes in assimilate partitioning and redirecting the flow of carbon away from sucrose biosynthesis toward amino acid synthesis.

A "futile" cycle of sucrose synthesis and degradation is involved in regulating partitioning between sucrose, starch and respiration in cotyledons of germinating Ricinus communis L. seedlings when phloem transport is inhibited

There was a dramatic alteration in the pattern of metabolism of [U(14)C]glucose by cotyledons of germinating Ricinus communis L. seedlings when phloem transport was inhibited by removing most of the hypocotyl and root. (i) Incorporation into sucrose was decreased two- to threefold, incorporation into starch was stimulated three- to sixfold, and there was a small increase of respiration, (ii) Pulse-chase experiments using (14)C and measurements of the total sucrose content revealed a rapid cycle of sucrose synthesis and degradation. When export is inhibited there is a two- to threefold inhibition of unidirectional sucrose synthesis and a three-fold stimulation of unidirectional sucrose degradation. As a result, the net flux switches from rapid net synthesis to slow net mobilisation of sucrose, (iii) The cotyledons contained adequate activities of sucrose synthase, acid and alkaline invertase and sucrose-phosphate synthase to catalyse the observed rate of sucrose breakdown and synthesis, respectively. The extracted activities of the degradative enzymes did not change after inhibiting phloem transport. The maximum activity of sucrose-phosphate synthase was also unaltered, but the activity measured in the presence of limiting substrates and phosphate was decreased twofold, indicating that sucrose-phosphate synthase has been deactivated by a mechanism analogous to that occurring in spinach leaves. (iv) The switch from sucrose export to starch synthesis when phloem transport was prevented was accompanied by only a small (20-50%) increase of the sucrose concentration in the cotyledons, no change of hexose-phosphates, an increase (16-70%) of fructose-1,6-bisphosphate and triosephosphate, and a small decrease (15-30%) of glycerate-3-phosphate, glycerate-2-phosphate and phosphoenolpyruvate. Fructose-2,6-bisphosphate and pyrophosphate doubled when 10 mM phosphate was included in the medium bathing the cotyledons, but not when phosphate was omitted (v) It is concluded that a futile cycle involving simultaneous synthesis and degradation of sucrose allows sucrose metabolism to respond in an extremely sensitive manner when phloem export is inhibited. There is a dramatic switch of flux through the sucrose pool, even though there are only marginal changes in the concentrations of sucrose and metabolites, or in the rate of respiration.

In vitro phosphorylation and inactivation of spinach leaf sucrose-phosphate synthase by an endogenous protein kinase

(1) Partially purified preparations of spinach (Spinacia oleracea L.) leaf sucrose-phosphate synthase (SPS) contain an endogenous protein kinase that phosphorylates and inactivates the enzyme with [gamma-32P]ATP. (2) The kinetic effect of phosphorylation is to alter affinities for substrates and the effector inorganic phosphate without affecting maximum velocity. (3) Two-dimensional peptide mapping of tryptic digests of in vitro labeled SPS yielded two phosphopeptides (designated sites 5 and 7). Labeling of the two sites occurred equally with time, and both correlated with inactivation. Maximum inactivation was associated with incorporation of 1.5 to 2.0 mol P/mol SPS tetramer, and about 70% of the phosphoryl groups were incorporated into one of the sites (phosphopeptide 7). (4) Phosphorylation and inactivation were strongly inhibited by NaCl, and the presence of salt alters some characteristics of the kinase reaction. In the absence of salt, the apparent Km for Mg.ATP was estimated to be 5 microM. (5) The dependence of the rate of phosphorylation on SPS concentration suggested that SPS and the protein kinase are distinct enzymes, but have some tendency to associate especially in the presence of ethylene glycol. (6) Ca2+/EGTA and polyamines have no effect on the rate of phosphorylation, whereas polycations (polylysine, polybrene and protamine) are inhibitory. (7) Of the metabolic intermediates tested, Glc 6-P inhibited phosphorylation and inactivation of the enzyme. The inhibition was not antagonized by inorganic phosphate, which suggests that Glc 6-P may be an effector of the kinase, rather than the target protein. Regulation by Glc 6-P may be of physiological significance.

Sucrose-phosphate synthase is dephosphorylated by protein phosphatase 2A in spinach leaves. Evidence from the effects of okadaic acid and microcystin

Sucrose-phosphate synthase (SPS) purified from spinach leaves harvested in the dark, was activated by mammalian protein phosphatase 2A (PP2A). Activation of SPS in a fraction from darkened spinach leaves was largely prevented by either okadaic acid or microcystin-LR (specific inhibitors of PPI and PP2A), while inhibitor-2 (a PP1 inhibitor) or Mg2+ (essential for PP2C) were ineffective. In vivo, okadaic acid and microcystin-LR prevented the light-induced activation of SPS and decreased sucrose biosynthesis and CO2 fixation. It is concluded that PP2A is the major SPS phosphatase in spinach. This study is the first to employ microcystin-LR for modulating protein phosphorylation in vivo.

Control of photosynthate partitioning in spinach leaves : Analysis of the interaction between feedforward and feedback regulation of sucrose synthesis

Experiments were carried out to estimate the elasticity coefficients and thence the distribution of control of sucrose synthesis and photosynthate partitioning between cytosolic fructose-1,6-bisphosphatase and sucrose-phosphate synthase (SPS), by applying the dualmodulation method of Kacser and Burns (1979, Biochem. Soc. Trans. 7, 1149-1161). Leaf discs of spinach (Spinacia oleracea L.) were harvested at the beginning and end of the photoperiod and illuminated at five different irradiances to alter (i) the extent of feedback inhibition and (ii) the rate of photosynthesis. The rate of CO2 fixation, sucrose synthesis and starch synthesis were measured and compared with the activation of SPS, and the levels of fructose-2,6-bisphosphate (Fru2,6bisP) and metabolites. Sucrose synthesis increased progressively with increasing irradiance, accompanied by relatively large changes of SPS activity and Fru2,6bisP, and relatively small changes of metabolites. At each irradiance, leaf discs harvested at the end of the photoperiod had (compared with leaf discs harvested at the beginning of the photoperiod) a decreased rate of sucrose synthesis, increased starch synthesis, decreased SPS activity, increased Fru2,6bisP, a relatively small (20%) increase of most metabolites, no change of the glycerate-3-phosphate: triose-phosphate ratio, a small increase of NADPmalate dehydrogenase activation, but no inhibition of photosynthesis. The changes of sucrose and starch synthesis were largest in low light, while the changes of SPS and Fru2,6bisP were as large, or even larger, in high light. It is discussed how these results provide evidence that the control of sucrose synthesis is shared between SPS and fructose-1,6-bisphosphatase, and provide information about the in-vivo response of these enzymes to changes in the levels of their substrates and effectors. At low fluxes, feedback regulation is very effective at altering partitioning. In high light, changes of SPS activation and Fru2,6bisP can be readily overriden by increasing levels of metabolites.

Species and Environmental Variations in the Effect of Inorganic Phosphate on Sucrose-Phosphate Synthase Activity : Reliability of Assays Based Upon UDP Formation

The effect of inorganic phosphate (Pi) on sucrose-phosphate synthase (SPS) activity was determined for the enzyme from five plant species (Nicotiana tabacum L., Spinacia oleracea L., Triticum aestivum L., Zea mays L., Glycine max L.) using two assay methods. The assay method based on determination of uridine diphosphate glucose- (UDPG) and fructose-6-phosphate-dependent sucrose formation was linear up to 15 minutes for all species tested. When assayed in this way, the effect of Pi at levels of 5 or 10 millimolar in the assay was variable, ranging from 0 to 35% inhibition of SPS activity. The assay method based on substrate dependent UDP formation was linear for some, but not for all of the species tested. Deviations from linearity were caused by loss of UDP from the assay medium. In some species, the extent of UDP loss was influenced by the level of Pi in the assay medium and, for at least one species (tobacco), it was influenced by the environment in which the plants were grown. The results indicated that (a) the role of Pi as an effector of SPS may vary depending on the species, and (b) the UDP assay method should be used with caution for assays of crude or desalted extracts, particularly when evaluating the effect of Pi on SPS activity.

Leaf Carbon Metabolism and Metabolite Levels during a Period of Sinusoidal Light

Photosynthesis rate, internal CO(2) concentration, starch, sucrose, and metabolite levels were measured in leaves of sugar beet (Beta vulgaris L.) during a 14-h period of sinusoidal light, which simulated a natural light period. Photosynthesis rate closely followed increasing and decreasing light level. Chloroplast metabolite levels changed in a manner indicating differential activation of enzymes at different light levels. Starch levels declined during the first and last 2 hours of the photoperiod, but increased when photosynthesis rate was greater than 50% of maximal. Sucrose and sucrose phosphate synthase levels were constant during the photoperiod, which is consistent with a relatively steady rate of sucrose synthesis during the day as observed previously (BR Fondy et al. [1989] Plant Physiol 89: 396-402). When starch was being degraded, glucose 1-phosphate level was high and there was a large amount of glucose 6-phosphate above that in equilibrium with fructose 6-phosphate, while fructose 6-phosphate and triose-phosphate levels were very low. Likewise, the regulatory metabolite, fructose, 2,6-bisphosphate was high, indicating that little carbon could move to sucrose from starch by the triose-phosphate pathway. These data cast doubt upon the feasibility of significant carbon flow through the triose-phosphate pathway during starch degradation and support the need for an additional pathway for mobilizing starch carbon to sucrose.

Light/Dark profiles of sucrose phosphate synthase, sucrose synthase, and Acid invertase in leaves of sugar beets

The activity of sucrose phosphate synthase, sucrose synthase, and acid invertase was monitored in 1- to 2-month-old sugar beet (Beta vulgaris L.) leaves. Sugar beet leaves achieve full laminar length in 13 days. Therefore, leaves were harvested at 2-day intervals for 15 days. Sucrose phosphate synthase activity was not detectable for 6 days in the dark-grown leaves. Once activity was measurable, sucrose phosphate synthase activity never exceeded half that observed in the light-grown leaves. After 8 days in the dark, leaves which were illuminated for 30 minutes showed no significant change in sucrose phosphate synthase activity. Leaves illuminated for 24 hours after 8 days in darkness, however, recovered sucrose phosphate synthase activity to 80% of that of normally grown leaves. Sucrose synthase and acid invertase activity in the light-grown leaves both increased for the first 7 days and then decreased as the leaves matured. In contrast, the activity of sucrose synthase oscillated throughout the growth period in the dark-grown leaves. Acid invertase activity in the dark-grown leaves seemed to be the same as the activity found in the light-grown leaves.

Phytochrome mediated regulation of sucrose phosphate synthase activity in maize

The extractable activity of sucrose phosphate synthase was determined in etiolated seedlings of maize (Zea mays L.), soybean (Glycine max [L.] Merr.), and sugar beet (Beta vulgaris L.) following treatments of changing light quality. A 30-minute illumination of 30 microeinsteins per square meter per second white light produced a three-fold increase in sucrose phosphate synthase activity at 2 hours postillumination when compared to seedlings maintained in total darkness. Etiolated maize seedlings treated with 3.6 microeinsteins per square meter per second of red and far-red light showed a 50% increase and a 50% decrease in sucrose phosphate synthase activity, respectively, when compared to etiolated maize seedlings treated with white light. Maize seedlings exposed for 30 minutes to red followed by 30 minutes to far-red showed an initial increase in sucrose phosphate synthase activity followed by a rapid decrease to control level. Neither soybean or sugar beet sucrose phosphate synthase responded to the 30-minute illumination of white light. Phytochrome is involved in sucrose phosphate synthase regulation in maize, whereas it is not responsible for changes in sucrose phosphate synthase activity in soybean or sugar beet.

Coarse control of sucrose-phosphate synthase in leaves: Alterations of the kinetic properties in response to the rate of photosynthesis and the accumulation of sucrose

It has been investigated whether diurnal rhythms of sucrose-phosphate synthase (SPS) are involved in controlling the rate of photosynthetic sucrose synthesis. Extracts were prepared from spinach (Spinacia oleracea L.) and barley (Hordeum vulgare L.) leaves and assayed for enzyme activity. The activity of SPS increased in parallel with a rising rate of photosynthesis, and was increased by feeding mannose and decreased by supplying inorganic phosphate. In leaf material where sucrose had accumulated during the photoperiod or when sucrose was supplied exogenously, SPS activity decreased. During a diurnal rhythm, SPS activity increased after illumination, declined gradually during the light period, decreased further after darkening and then recovered gradually during the night. These changes did not involve an alteration of the maximal activity, but were caused by changes in the kinetic properties, revealed as a change in sensitivity to inhibition by inorganic phosphate. In experiments which modelled the response of SPS to changing metabolite concentrations, it was shown that these alterations of kinetic properties would strongly modify the activity of SPS in vivo. It is proposed that SPS can exist in kinetically distinct forms in vivo, and that the distribution between these forms can be rapidly altered. As the rate of photosynthesis increases there is an activation of SPS, which may be directly or indirectly linked to changes in the availability of Pi. This activation can be modified by factors related to the accumulation of sucrose. Under normal conditions there is a balance between these factors, and the leaf contains a mixture of the different forms of SPS.

Light Modulation and Localization of Sucrose Phosphate Synthase Activity between Mesophyll Cells and Bundle Sheath Cells in C(4) Species

Experiments were conducted with several Panicum species (representing the different C(4) subtypes) to examine the light modulation of sucrose phosphate synthase (SPS) activity and the effect of illumination on the distribution of SPS activity between mesophyll cells (MC) and bundle sheath cells (BSC). Activity of SPS in the light decreased in the order: C(4) > C(3)-C(4) intermediate > C(3). In illuminated leaves, SPS activities were similar among the three C(4) subtypes, but SPS activity was higher for NAD-malic enzyme (NAD-ME) species with centripetal chloroplasts in BSC (NAD-ME(P) species) than for NAD-ME species with centrifugal chloroplasts in BSC (NAD-ME(F) species). Transfer of plants into darkness for 30 minutes resulted in decreased SPS activity for all species tested except Panicum bisulcatum (C(3) species) and Panicum virgatum (NAD-ME(P) species) which showed little or no change. All C(4) subtypes had some SPS activity both in MC and BSC. In the light, SPS activity was mainly in the MC for NADP-ME, NAD-ME(F) and phosphoenolpyruvate carboxykinase species, while it was mainly in the BSC for NAD-ME(P) species. In the dark, for all C(4) subtypes, SPS activity in the MC was decreased to a greater extent than that in the BSC. It is intriguing that NAD-ME(F) and NAD-ME(P) species differed in the activity and distribution of SPS activity between MC and BSC, although they are otherwise identical in the photosynthetic carbon assimilation pathway. Diurnal changes in SPS activity in the MC and BSC were also examined in maize leaves. SPS activity in the MC in maize leaves was high and relatively constant throughout the middle of the light period, dropped rapidly after sunset and increased again prior to the light period. On the other hand, SPS activity in the BSC was lower and changed more coincidently with light intensity than that in the MC. The results suggested that light activation of SPS activity located in the BSC may require higher irradiance for saturation than the SPS in the MC. We conclude that SPS may function in both MC and BSC for sucrose synthesis in the light, particularly at high light intensity, while in the dark, the major function may be in the BSC during starch degradation.

Diurnal changes in maize leaf photosynthesis : I. Carbon exchange rate, assimilate export rate, and enzyme activities

Diurnal changes in photosynthetic parameters and enzyme activities were characterized in greenhouse grown maize plants (Zea mays L. cv Pioneer 3184). Rates of net photosynthesis and assimilate export were highest at midday, coincident with maximum irradiance. During the day, assimilate export accounted for about 80% of net carbon fixation, and the maximum export rate (35 milligrams CH(2)O per square decimeter per hour) was substantially higher than the relatively constant rate maintained through the night (5 milligrams CH(2)O per square decimeter per hour). Activities of sucrose phosphate synthase and NADP-malate dehydrogenase showed pronounced diurnal fluctuations; maximum enzyme activities were generally coincident with highest light intensity. Reciprocal light/dark transfers of plants throughout the diurnal cycle revealed that both enzymes were deactivated by 30 minutes of darkness during the day, and they could both be substantially activated by 30 minutes of illumination at night. During 24 hours of extended darkness, sucrose phosphate synthase activity declined progressively to an almost undetectable level, but was activated after 1.5 hours of illumination. Thus, the diurnal fluctuation in maize sucrose phosphate synthase can be explained by some form of light modulation of enzyme activity and is not due to an endogenous rhythm in activity. No diurnal fluctuations were observed in the activities of NADP-malic enzyme or fructose 6-phosphate-2-kinase. Phosphoenolpyruvate carboxylase was activated by light to some extent (about 50%) when activity was measured under suboptimal conditions in vitro. The results suggested that the rates of sucrose formation and assimilate export were closely aligned with the rate of carbon fixation and the activation state of sucrose phosphate synthase.

Subcellular Metabolite Levels in Spinach Leaves : Regulation of Sucrose Synthesis during Diurnal Alterations in Photosynthetic Partitioning

The alterations of subcellular metabolite levels during the day in spinach leaves have been investigated using nonaqueous density gradient centrifugation to separate chloroplasts, cytosol, and vacuole. The results provide direct evidence for the role of sucrose phosphate synthase and cytosolic fructose 1,6-bisphosphatase in regulating sucrose synthesis in leaves and also show that the phosphate translocator is kinetically limiting in vivo.

Control of photosynthetic sucrose synthesis in barley primary leaves: role of fructose 2,6-bisphosphate

Levels of fructose 2,6-bisphosphate (F2,6BP) and related metabolites were measured in 8- or 9-day-old barley (Hordeum vulgare L.) primary leaves throughout a 24 hour cycle. Young barley leaves contained about 0.4 nanomole F2,6BP per milligram chlorophyll at the end of a 12 hour dark period. F2,6BP levels increased rapidly following a dark-to-light transition and then decreased to about 0.1 nanomole per milligram chlorophyll after 5 or 10 minutes of light. Low levels of F2,6BP were detected in barley primary leaves throughout the day. A 10-fold increase in F2,6BP was observed during the first hour of the dark period and then levels of this metabolite decreased slowly for the next several hours. Only small diurnal fluctuations were noted in barley leaf glucose 6-phosphate and uridine 5'-diphosphoglucose levels. There were rapid changes in whole leaf F2,6BP levels when the light intensity was altered. High F2,6BP levels in the dark were not observed after short photosynthetic periods. Results obtained with barley primary leaves support the suggestion that F2,6BP is involved in regulating the flow of photosynthate from the chloroplast to sucrose. Extractable sucrose-phosphate synthase activity was inversely related to barley primary leaf F2,6BP levels. This finding may indicate that the activities of sucrose-phosphate synthase and cytosolic fructose 1,6-bisphosphatase in barley primary leaves are metabolically coordinated.

Diurnal fluctuations in cotton leaf carbon export, carbohydrate content, and sucrose synthesizing enzymes

In fully expanded leaves of greenhouse-grown cotton (Gossypium hirsutum L., cv Coker 100) plants, carbon export, starch accumulation rate, and carbon exchange rate exhibited different behavior during the light period. Starch accumulation rates were relatively constant during the light period, whereas carbon export rate was greater in the afternoon than in the morning even though the carbon exchange rate peaked about noon. Sucrose levels increased throughout the light period and dropped sharply with the onset of darkness; hexose levels were relatively constant except for a slight peak in the early morning. Sucrose synthase, usually thought to be a degradative enzyme, was found in unusually high activities in cotton leaf. Both sucrose synthase and sucrose phosphate synthetase activities were found to fluctuate diurnally in cotton leaves but with different rhythms. Diurnal fluctuations in the rate of sucrose export were generally aligned with sucrose phosphate synthase activity during the light period but not with sucrose synthase activity; neither enzyme activity correlated with carbon export during the dark. Cotton leaf sucrose phosphate synthase activity was sufficient to account for the observed carbon export rates; there is no need to invoke sucrose synthase as a synthetic enzyme in mature cotton leaves. During the dark a significant correlation was found between starch degradation rate and leaf carbon export. These results indicate that carbon partitioning in cotton leaf is somewhat independent of the carbon exchange rate and that leaf carbon export rate may be linked to sucrose formation and content during the light period and to starch breakdown in the dark.

Possible control of maize leaf sucrose-phosphate synthase activity by light modulation

Sucrose phosphate synthase (SPS) activity was measured in extracts of maize (Zea mays L.) and soybean (Glycine max L. [Merr.]) leaves over a single day/night cycle. There was a 2- to 3-fold postillumination increase in extractable enzyme activity in maize leaves, whereas the activity of soybean SPS was only about 30% higher in extracts prepared from light- compared to dark-adapted leaves. Alterations in extractable maize leaf SPS activity correlated with light/dark transitions suggesting that the enzyme may be light modulated. Diurnal variations of extractable maize leaf SPS activity were also observed in a greenhouse experiment. A transition from high (light) to low (dark) extractable SPS activity occurred near the light compensation point for photosynthesis (about 20 micromole photons per square meter per second). Further increases in irradiance did not increase extractable SPS activity. Substrate affinities for uridine 5'-diphosphoglucose (Michaelis constant = 3.5 and 5.1 millimolar) and fructose-6 phosphate (half maximal concentration = 1.0 and 2.5 millimolar) were lower for partially purified SPS obtained from light compared to dark acclimated maize leaves. Light-induced changes in extractable SPS activity were stable for at least one column chromatography step. The above results indicate that light-induced changes in SPS activity may be important in controlling the photosynthetic production of sucrose.

Sucrose and Starch Synthesis in Spinach Plants Grown under Long and Short Photosynthetic Periods

The flow of carbon into sucrose and starch was investigated in fully expanded primary leaves of spinach using the long to short day transition and partial defoliation as tools to manipulate sucrose/starch synthesis. Transfer from 12 hour to 7 hour photosynthetic periods resulted in a 4-fold increase in the initial rate of starch synthesis, a 50% increase in the initial rate of sucrose synthesis, a 30% increase in leaf sucrose, and a 40% decrease in fructose, 2,6-biphosphate. In addition, sucrose synthesis rates in cells isolated from shortened daylength plants are 80% higher than in cells isolated from control plants. These results show that, in spinach, an increase in the rates of both sucrose and starch synthesis can occur under short day conditions. In contrast, when short day plants are partially defoliated, starch levels remain high, fructose 2,6-biphosphate levels remain low, but the level of leaf sucrose drops by 50%. Thus, when demand exceeds supply, starch synthesis has priority over filling of leaf sucrose pools in the short day plant.